Bones point towards ground
You'll have to utilize more bones and constraints for this
I'm not sure if this is exactly what you are looking for but maybe it can be a starting point. 

The toe bones have a "Locked To" constraint which are tracking the "track_to" bone, I set up a foot bone which is what you will animate and the "track_to" bone will follow the foot bone along the ground which has "Copy Location" constraint on it. Download the Blend if you want to deconstruct the setup. 



            

Bonus Gif